LEETCODE: Gas Station

There are N gas stations along a circular route, where the amount of gas at station i is 

gas[i]

.

You have a car with an unlimited gas tank and it costs 

cost[i]

 of gas to travel from station i to
its next station (i+1). You begin the journey with an empty tank at one of the gas stations.

Return the starting gas station's index if you can travel around the circuit once, otherwise return -1.

Note:

The solution is guaranteed to be unique.

class Solution {
public:
int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
if(gas.size() != cost.size()) return -1;
int start = 0;
int total = 0;
int lacking = 0;
for(int ii = 0; ii < gas.size(); ii ++) {
total += gas[ii] - cost[ii];
if(total < 0) {
lacking += total;
start = ii + 1;
total = 0;
}
}
if(start >= gas.size())
return -1;
return total + lacking >= 0 ? start : -1;
}
};